oK im sorry , i missed that it was a breechloader .
Once the newer powders came out , most manufactures started building to ensure proof for that higher standards . Thus its really a greater is good for lesser type of thing . IE a gun proofed for smokeless would be also capable of shooting BP . So the point could be made that the manufacture of guns for BP never stopped. BP cartridges just became harder to find

i would stretch to guess that for BP proof it would be sometime in the very early 20th century.
I say that because by the late 19th century wood powder was in common use for cartridge shot guns .

Damascus wasn’t made just for black powder it was also used with smokeless and proofed for such .
It can be found on both box lock configurations and hammer style locks .
So he may very well have a rabbit ear gun with a pattern steel barrel that has smokeless proof markings .
In fact you can still today get pattern steel barrels that are proofed to for even magnum rifle loads.
Its been a couple years but last I checked it was running around 50.00 US per inch for a barrel blank

It should also be noted that a lot of what people call Damascus is in fact not Damascus at all
Laminated steel barrels and all those within the laminated class like stub twist , English twist and Belgium twist are not true Damascus
Neither is fluid steel
The other thing is that a proof mark is not a manufactures mark . A barrel can carry an early manufacture mark and yet still have a later proof mark . It can also carry more then one proof if the barrels were subject to a reproof at a later date .
There is this common misconception that just because a barrel carries a given proof mark that its ok to shoot .
What a proof mark means is that at the time the barrels were submitted , they past proof . Yet the gun can go out of proof very easily. An over load , improper care … all can take a gun out of proof . As such the simple reality is that most of the guns people shoot today that carry a proof mark would be considered out of proof for any number of reasons

Also we should remember that just because a barrel may carry a smokeless proof mark , that doesn’t mean it was proofed to the level of today’s smokeless powders . Thus a lot of us who chose to shoot these old guns , chose to use a low base powder if not BP no mater if they are Damascus barrels or steel barrels doesn’t mater if they are parkers , Colts , Remington’s , Greeners …..
I still own several ,Victor , Fox , Parker and a couple Cresants . All but 1 are hammer guns

Also it is rather funny that Belgium / liege barrels get such a bad rap .
Yet often times many of what we consider very good guns , carry Belgium made barrels
But what a lot of people don’t realize is that Parker ,Remington, Ithaca, Colt , L.C . Smith , Baker ……. All were using Belgium barrels and said so during a congressional hearings in 1912 concerning a congressional report on duties on metals an manufacturing

Indy , here is a link that will help you start to research your Belgium gun .
It carries a lot of information on bimanous and the different makers as well as proof markings that can be found on Damascus or patern barrels , both smokeless and black

also if you look on this list under ; Belgium shotgun identification and makers marks
You will find some very interesting reading and dates as well as key information on discerning smokeless proof markings